docs: def _to_ned(self):
          """
          Switches the reference frame to NED
          """
          if self.ref_frame is 'USE':
              # Rotate
              return utils.use_to_ned(self.tensor), \
                  utils.use_to_ned(self.tensor_sigma)
          elif self.ref_frame is 'NED':
              # Alreadt NED
              return self.tensor, self.tensor_sigma
          else:
              raise ValueError('Reference frame %s not recognised - cannot '
                               'transform to NED!' % self.ref_frame)